1. Field of the Invention
This invention relates generally to power generation systems and, more particularly, this invention relates to electrochemical cells and methods for controlling and improving their performance thereof.
2. Description of Related Art
Electrochemical cells utilizing a reactive metal anode, an electrically conductive cathode, and an aqueous electrolyte are well known. Such cells are described in detail in numerous patents and publications, including Rowley U.S. Pat. No. 3,791,871 (Feb. 12, 1974), the disclosure of which is incorporated herein by reference.
The cell disclosed in the Rowley patent typifies prior electrochemical cells and utilizes a metal anode which is highly reactive with water and spaced from a cathode by an electrically insulating film formed on the anode in the presence of water. The anode and the cathode are not in direct electrical contact with each other but are in contact with an aqueous electrolyte during operation of the cell. In addition, circuit connections are made at each electrode for drawing electric power from the cell. In the cell of the Rowley patent, the anode comprises an elemental alkali metal, such as sodium or lithium, and the electrolyte comprises an aqueous solution of sodium hydroxide or lithium hydroxide, respectively, produced by the electrochemical reaction of the anodic metal with water.
In such cells, the hydroxide electrolyte concentration is a control parameter for the rate of reaction of the water of the electrolyte and the anode metal. In the past, the power output from such electrochemical cells has been controlled primarily by adjustments made to the concentration of the electrolyte flowing through the cell, with the electrolyte concentration being controlled to prevent anode melting and/or thermal runaway. Typically, such control is effected by the continuous addition of water to the electrolyte while electric power is simultaneously drawn from the cell. The rate of the water addition is appropriately restricted so as to replace the water reduced in the cell during operation and also to maintain the electrolyte concentration balance. A water inlet rate which will result in satisfactory cell operation may be readily established empirically or calculated by material balance.
Since flooding of such a cell with water can result in thermal runaway and explosion, a critical design consideration in such controlled power generation systems is to limit the maximum rate at which water can enter the cell. However, in some applications, limiting the maximum rate at which water can enter the cell may not be feasible and/or practical.
Furthermore, control of such cells so as to obtain substantially flat voltage profiles is desired because electrical motors are commonly regulated via the regulation of the voltage generated thereby. Therefore, systems and methods by which electrochemical cells can be operated for extended periods of time with substantially flat voltage profiles have been sought.